182
J. w. Morse
7.3.1.3
Carbonate Precipitation in Sulphidic Sediments
In Fig. 7.3, it was shown that sulphate reduction that initially results in a major decrease
in pore water saturation with respect to calcium carbonate can lead again to supersaturation with increasing extent of sulphate reduction. In sediments and sedimentary rocks, this is a well-known mechanism for producing authigenic calcium carbonate.
For example, the massive carbonate cap rocks on salt domes are a result of this process.
An example from modern sediments comes from Baffin Bay, Texas, which is a hypersaline negative estuary that has been studied because of its similarities to environments that may have existed in ancient epicontinental seas (Morse et al. 1992). Figure 7.11 shows the relationship of the loss of dissolved sulphate and calcium to the increase in total carbon dioxide. The decrease in dissolved calcium occurs because of
the precipitation of calcium carbonate. If the loss of carbon dioxide from precipitation is corrected for by the loss of calcium, then a close to ideal two to one increase in
dissolved carbon dioxide with sulphate loss is observed (Fig. 7.12).
For the precipitation of calcium carbonate to be quantitatively significant, it is necessary for there to be exceptionally high quantities of metabolizable organic matter
available to produce the needed carbon dioxide. An example of such a situation is at
hydrocarbon seeps in the Gulf of Mexico (Arvidson and Morse, to be published). There,
the organic content of the sediments and concentration of calcium carbonate show a
close association. (Fig. 7.13). Because large amounts of sulphide are also produced by
this process, total dissolved hydrogen sulphide can exceed 10 mM.
7.3.2
Carbonate-Rich Sediments
During the last decade, Lynn Walter and her associates (Walter and Burton 1990; Walter
et al. 1993; Ku et al. 1999) have done much to demonstrate the major quantitative importance of dissolution of calcium carbonate from shallow carbonate sediments (on
the order of up to 50%) driven by the previously discussed sulphate reduction-sulphide oxidation process. Such sediments account for about a third of all ocean carbonate production (Milliman 1993) and are therefore of considerable significance in
understanding the global carbon cycle and impact of fossil fuel CO 2 ,
The basic "problem" for the sulphide system in calcium carbonate-rich sediments
is the general lack of reactive iron to produce iron sulphide minerals. The sulphide
that is produced by sulphate reduction then basically can only be buried in dissolved
form in pore waters, be oxidized or diffuse out the sediments. For most carbonaterich sediments, the oxidative process strongly dominates the fate of the sulphide. This
is not grossly unlike what also happens in "normal" marine sediments. However, no
sulphide minerals are produced, and since these sediments are carbonate-rich they
will remain relatively close to equilibrium between calcium carbonate and their pore
waters (see Fig. 7.4; Morse et al. 1985).
Figure 7.14 (Walter et al. 1993) shows the strong relationship that generally occurs
in carbonate muds from Florida Bay between total carbon dioxide, excess dissolved
calcium and the amount of sulphate that has been reduced. This figure is similar to
Fig. 7.11, except instead of calcium decreasing, it increases indicating dissolution rather
J. w. Morse
7.3.1.3
Carbonate Precipitation in Sulphidic Sediments
In Fig. 7.3, it was shown that sulphate reduction that initially results in a major decrease
in pore water saturation with respect to calcium carbonate can lead again to supersaturation with increasing extent of sulphate reduction. In sediments and sedimentary rocks, this is a well-known mechanism for producing authigenic calcium carbonate.
For example, the massive carbonate cap rocks on salt domes are a result of this process.
An example from modern sediments comes from Baffin Bay, Texas, which is a hypersaline negative estuary that has been studied because of its similarities to environments that may have existed in ancient epicontinental seas (Morse et al. 1992). Figure 7.11 shows the relationship of the loss of dissolved sulphate and calcium to the increase in total carbon dioxide. The decrease in dissolved calcium occurs because of
the precipitation of calcium carbonate. If the loss of carbon dioxide from precipitation is corrected for by the loss of calcium, then a close to ideal two to one increase in
dissolved carbon dioxide with sulphate loss is observed (Fig. 7.12).
For the precipitation of calcium carbonate to be quantitatively significant, it is necessary for there to be exceptionally high quantities of metabolizable organic matter
available to produce the needed carbon dioxide. An example of such a situation is at
hydrocarbon seeps in the Gulf of Mexico (Arvidson and Morse, to be published). There,
the organic content of the sediments and concentration of calcium carbonate show a
close association. (Fig. 7.13). Because large amounts of sulphide are also produced by
this process, total dissolved hydrogen sulphide can exceed 10 mM.
7.3.2
Carbonate-Rich Sediments
During the last decade, Lynn Walter and her associates (Walter and Burton 1990; Walter
et al. 1993; Ku et al. 1999) have done much to demonstrate the major quantitative importance of dissolution of calcium carbonate from shallow carbonate sediments (on
the order of up to 50%) driven by the previously discussed sulphate reduction-sulphide oxidation process. Such sediments account for about a third of all ocean carbonate production (Milliman 1993) and are therefore of considerable significance in
understanding the global carbon cycle and impact of fossil fuel CO 2 ,
The basic "problem" for the sulphide system in calcium carbonate-rich sediments
is the general lack of reactive iron to produce iron sulphide minerals. The sulphide
that is produced by sulphate reduction then basically can only be buried in dissolved
form in pore waters, be oxidized or diffuse out the sediments. For most carbonaterich sediments, the oxidative process strongly dominates the fate of the sulphide. This
is not grossly unlike what also happens in "normal" marine sediments. However, no
sulphide minerals are produced, and since these sediments are carbonate-rich they
will remain relatively close to equilibrium between calcium carbonate and their pore
waters (see Fig. 7.4; Morse et al. 1985).
Figure 7.14 (Walter et al. 1993) shows the strong relationship that generally occurs
in carbonate muds from Florida Bay between total carbon dioxide, excess dissolved
calcium and the amount of sulphate that has been reduced. This figure is similar to
Fig. 7.11, except instead of calcium decreasing, it increases indicating dissolution rather